I am arriving on a Sunday and need Zones 1-3 coverage through the following Sunday. Can I buy Navigo Decouverte (to cover Mon-Sun) and add a Navigo Day Pass to cover the Sun I arrive? Or should I just get a one-day Visite Pass to cover the first day?
Yes you certainly can add a Navigo Jour day pass 1-3 zone for your Sunday/first day and also add a Navigo week pass for the Monday-Sunday to follow, all during the same transaction at a ticket window / ticket office. You can avoid carrying the paper ticket Paris Visite this way, plus the Navigo day pass is slightly cheaper at 10€ vs. 12€ for a 1-3 zone 1 day Paris Visite.